Magnetic resonance imaging (MRI) has emerged as a very important technique in medical diagnostics and more recently in the study of complex biological processes such as gene expression (Cheery, 2004, Phy. In Med. Biol. 49:R13-48). Magnetic resonance imaging affords many unique advantages because it provides data in four dimensions (x, y and z axis, and time) and also permits non-invasive imaging of whole animals. One limitation of MRI, however, is its relatively low sensitivity, which is typically addressed with the use of contrast agents (CAs). Current clinically used CAs are small molecules, typically paramagnetic ions (e.g., gadolinium(III)) chelated by small organic molecules, and are used as blood pool agents that are excreted from the body in less than an hour. Recent research has turned toward synthetic polymers to increase the blood retention time of CAs, but such polymers are generally polydisperse and hence difficult to characterize and control (Huber et. al., 1998, Conj. Chem. 9:242-249; Watanabe, 2002, Mag. Res. Med. Sci. 1:38-49).
With increasing focus on earlier detection of disease, MRI is poised to play an important role in predictive and preventative medicine. Magnetic resonance imaging non-invasively provides three-dimensional visualization of whole tissues as well as molecular targets and processes at cellular resolution (˜10 μm) (Huber et al., 1998). For example, the progression of tumors can be assessed through MRI by detection of tumor vascular permeability measurements (Wen et al., 2004, Bioconj. Chem. 15:1408-1415). Besides screening and staging of disease, MRI can also be used to select and optimize therapy (Artemov et al., 2004, Curr. Pharm. Biotech. 5:485-494).
In addition to improving medical diagnostics and therapeutics, MRI has great potential for enabling new discoveries in cellular and molecular biology. The imaging of gene expression commonly uses an optically visible reporter that is co-expressed with the gene of interest. In order to analyze the reporter, these methods often require killing the animal and fixing the tissue for histochemical detection (Genove et al., 2005, Nat. Med. 11:450-454). Magnetic resonance imaging also has significant advantages over other techniques in detecting cell signaling. Light microscopy, the most popular method of imaging cell signaling, is limited by light scattering, often produces photobleaching byproducts, and is also invasive (Lee et al., 2005, J. Am. Chem. Soc. 127:13164-13166). Imaging techniques such as PET and SPECT require the use of radioactive tracers and additionally are limited by low spatial resolution (Artemov et al., 2004). By preserving the animal, MRI allows for the examination of complex biological processes over time and in a variety of conditions.
Magnetic resonance imaging takes advantage of the interaction between an applied magnetic field and the nuclear spin of hydrogen protons (Brown and Semelka, 2003, MRI: Basic Principles and Applications, 3rd ed., John Wiley & Sons, Inc, Hoboken, N.J.). Instruments used for MRI measure variations in the excitation (or relaxation) of protons associated with water molecules to produce an image, the intensity of which is related to water concentration and the rate of relaxation of proton spins. The relaxation time T1, the spin-lattice relaxation time, is a measure of how quickly protons release energy to the surroundings following an excitation pulse (Brown and Semelka, 2003). The spin-spin relaxation time, T2, is the time for proton-proton energy transfer. A shorter T1 relaxation time leads to a stronger net magnetization of the protons, providing a higher intensity signal. On the other hand, faster T2 relaxation dephases the nearby water protons more than the surrounding tissue, resulting in signal loss and better negative contrast (Brown and Semelka, 2003).
The primary limitation of MRI is the lack of sufficient contrast difference between the water proton signal in the area of interest and the surrounding tissues. To overcome this problem, contrast agents are employed to increase the relaxation difference. Contrast agents (T1) provide positive contrast enhancement by employing paramagnetic ions, such as gadolinium (III) (Gd (III)), manganese (II) (Mn(II)), and iron (III) (Fe(III)) (Artemov et al., 2004). These ions have a high magnetic moment because of their large number of unpaired electrons, and can speed up the energy transfer to produce a faster relaxation rate. In currently used T1 contrast agents, a paramagnetic ion, which is toxic on its own, is chelated by small organic molecules such as diethylenetriaminepentaacetic acid (DTPA) and 1,4,7,10-tetraazacyclododecome-1,4,7,10-tetraacetic acid (DOTA) (Wang et al., 2004, Pharm. Res. 22:596-602). Magnevist™ and Omniscan™ are two frequently used commercial products for MRI that contain Gd(III) chelated by DTPA (Artemov et al., 2004). However, these low-molecular weight agents not only have low retention times in vivo, but they also provide insufficient contrast to allow MRI methods to be applied in all of the potential medical diagnostic and biological research applications which are of interest.
Macromolecular scaffolds for the display of multiple Gd(III) chelating moieties have been used to improve the performance of contrast agents. By coupling multiple paramagnetic ion chelators to macromolecules, both the circulation time and the image contrast are increased. A longer image acquisition time that allows multiples scans, an increase in the paramagnetic ion concentration, and a larger rotational correlation time associated with macromolecules all serve to enhance the signal-to-noise ratio for higher contrast and resolution (Wen et al., 2004; Lee et al., 2005). The rotational correlation time, which is related to the time for rotation of the molecule, greatly contributes to the overall relaxation rate, with greater rotational correlation times improving image contrast (Caravan et al., 1999, Chem. Rev. 99:2293-2352). Several different macromolecular scaffolds have been used in MRI agents. Synthetic polymers have been employed both as linear polymers, most commonly polylysine (Opsahl et al., 1995, Acad. Rad. 2:762-767) and polyethylene glycol (PEG) (Desser et al., 1994, J. Mag. Res. Imaging 4:467-472), and as dendrimers (Kobayashi and Brechbiel, 2005, Ad. Drug Del. Rev. 57:2271-2286; Langereis et al., 2006, NMR in Biomed. 19:133-141). Polyethylene glycol chains have been added to other polymers to modulate the pharmacokinetic properties of the contrast agents since PEG shields the macromolecule and increases retention time (Caravan et al., 1999).
In contrast to synthetic linear polymers that are generally polydisperse, dendrimers are highly branched polymers that are almost monodisperse. The high density of end groups provides many sites for chelator conjugation, which can increase the relaxivity for greater contrast. Furthermore, the rigid structure and shape of dendrimers increases the overall tumbling of the molecule for a larger rotational correlation time (Caravan et al., 1999). Although there have been improvements in syntheses of dendrimers, their preparation still require labor-intensive, multi-step processes. Naturally occurring polymers of sugars and polypeptides, which can be degraded in the body, have been investigated as macromolecular CA backbones. Dextran, which consists of glucose subunits, has been used as a blood pool agent (Sirlin et al., 2004, Acad. Rad. 11:1361; Huber et al., 1998). However, dextran is a polydisperse material, which has limited solubility in some preparations, causing difficulty in the preparation and characterization of the contrast agents.
Proteins such as human or bovine serum albumin (HSA or BSA) (Wang et al., 2004; Caravan et al., 1999), avidin (Langereis et al., 2004, Org. Bio. Chem. 2:1271-1273), and monoclonal antibodies (Shahbazi-Gahrouei et al., 2001, J. Mag. Res. Imaging 14:169-174) have been explored as contrast agents. Human or bovine serum albumin based contrast agents are used as “gold standard” blood pool agents since they are able to distinguish between benign and malignant tumors based on hyperpermeability of tumor vasculature (Caravan et al., 1999; Wang et al., 2004).
Liposomes are other natural macromolecules that can act as contrast agents. They can encapsulate gadolinium chelators in the aqueous compartment of a lipid bilayer coated vesicle, or chelators can be conjugated to the lipids that form the bilayer (Strijkers et al., 2005, Magma 18:186-192; Lanza et al., 2004, Curr. Pharma. Biotech. 5:495-507). However, these can be restricted to diffusion of water with liposomes, limiting proton exchange and therefore, image contrast.
Gadolinium (III) ions are toxic to a biological system on their own, but their strong chelation by an organic molecule prevents the body from the harmful effects of the metal ion. Low-molecular weight contrast agents pose very little safety risk and are widely known to be among the safest drugs that have ever been introduced (Caravan et al., 1999). Their efficient secretion from the body minimizes exposure of the contrast agent and limits the chance of uptake into cells. Furthermore, these low molecular weight contrast agents are secreted unaltered and intact, so the free organic molecule and free metal ions are not circulating in the body. In contrast, there are safety concerns with slow excretion of macromolecular Gd(III) complexes that may result in the build-up of toxic Gd(III) ions (Lu et al., 2003, Mag. Res. Med. 51:27-34). By virtue of their size, macromolecular contrast agents dwell longer in the body and they are not excreted as completely. Metabolism and partial degradation of the macromolecular contrast agents could potentially result in the release of free Gd(III) ions; degradation in low-pH lysosomes can cause the paramagnetic ion to dissociate from the chelator, resulting in free Gd(III). The macromolecular dendrimeric contrast agents have the advantage of extremely narrow molecular weight distribution, but they are not biodegradable (Wen et al., 2004). Additionally, large multivalent molecules are more likely to be antigenic than small molecules, leading to antibodies and potentially anaphylactic shock (Caravan et al., 1999). Serum albumin-based contrast agents particularly pose the threat of an immunogenic response (Wen et al., 2004).
To lower toxicity while maintaining the improved image intensity that comes from a macromolecular scaffold, researchers have designed rapidly biodegradable polymer-based contrast agents. One group has created polydisulfide-based biodegradable macromolecular Gd(III) complexes that can be cleaved through the disulfide groups into smaller complexes that are more readily eliminated from the body though renal filtration (Mohs et al., 2004, Bioconj. Chem. 15:1424-1430; Zong et al., 2005, Meg. Res. Med. 53:835-842; Wang et al., 2004). Another group designed and evaluated poly(L-glutamic acid)-based (PGA-based) contrast agents, since PGA is readily degraded by lysosomal enzymes to L-glutamic acid, a nontoxic natural compound (Wen et al., 2004).
Regenerating tissues is a challenging, important area in medicine that is a potential therapy for a vast number of diverse diseases. Because of the three-dimensional and non-invasive advantages as well as the ability to provide cellular resolution, MRI is an ideal imaging tool to track the fate of tissue engineering scaffolds and cells, cellular remodeling, angiogenesis, and other important parameters. Researchers have performed MRI with Gd-DTPA contrast agents to study various aspects of tissue engineering in bladders (Chang et al., 2005, J. Mag. Res. Imaging 21:415-423), meniscal cartilage (Neves et al., 2006, Tissue Eng. 12:53-62), and the heart (Barbash et al., 2004, Heart 90:87-91). The scaffold is a key part of tissue engineering, providing physical support and chemical signals for cellular growth. Mechanical properties, degradation rate, porosity, chemical composition, incorporated bioactive factors and other scaffold variables together determine the success of growing cells. Thus, there has been significant research using MRI to study various implantable biomaterials that act as scaffolds (Barbash et al., 2004; Neves et al., 2006; Stroman et al., 1999, Mag. Res. Med. 42:210-214; Guidoin et al., 2004, Art. Cells, Blood Subs., 1 mm. Biotech. 32:105-127; Pihlajamaki et al., 1997, Biomat. 18:1311-1315). Additionally, peptide amphiphile contrast agent molecules have been created that attach contrast agents to biomaterials that act as a tissue engineering scaffold, enabling imaging of the scaffold (Bull et al., 2005, Bioconj. Chem. 16:1343-1348; Bull et al., 2005. Nano Lett. 5:1-4).
In addition to increasing contrast, there has been work done on targeting MRI agents to particular tissues. Generally, MRI contrast agents are injected systemically and passively, and non-specifically enhance the MRI signal. There are blood pool agents that highlight the vasculature in normal and cancerous tissues. On the other hand, targeted contrast agents can be modified so that they localize to a specific tissue or cell type. A variety of ligands with high affinity and specificity to their receptor have been coupled to contrast agents for active targeting. Monoclonal antibodies or their fragments (Morawski et al., 2005, Curr. Opin. Biotech. 16:89-92), peptides, peptidomimetics, and aptamers are among the ligands that can be coupled covalently or non-covalently (Lanza et al., 2004). Receptors HER-21/neu, which are expressed on the surface of malignant breast cancer cells, were imaged using a biotinylated herceptin antibody and avidin-conjugated contrast agent (Artemov et al., 2004). The RGD epitope, which mimics the fibrinogen binding site to the αvβ3 integrin, has been incorporated into MRI contrast agents to image angiogenesis (Mulder et al., 2005, FASEB J. 19:2008-2010). In addition to targeting extracellular components, there is great interest in designing contrast agents that can cross the cellular membrane to allow intracellular imaging. There are known short peptides (less than 30 residues) with translocation properties, including the tat peptide from the HIV tat protein, polyarginine, and penetratrin (Lanza et al., 2004). Investigations with peptides showed that the role of positive charges is critical in effectively crossing cell membranes. Mitchell et al., 2000, J. Peptide Res. 56:318-325 showed that polyarginine containing six or more amino acids crosses cell membranes far more effectively than histidine, ornithine, or lysine polymers of equal length or longer. Both the tat protein (Wunderbaldinger et al., 2002, Bioconj. Chem. 13:264-268) and polyaxginine (Allen and Meade, 2003, J. Biol. Inorg. Chem. 8:746-750; Allen et al., 2004, Chem. Biol. 11:301-307) have been covalently attached to contrast agent platforms and shown to permeate cell membranes and improve intracellular MRI contrast. Additionally, steroid hormones have been conjugated to contrast agents to image intracellular signaling pathways (Lee et al., 2005). However, the aforementioned conjugated contrast agents are time consuming and inefficient to produce, as well as being polydisperse for limited gadolinium chelating.
As such, what are needed are compositions and methods for use in magnetic resonance imaging that not only increase imaging sensitivity and contrast, but also can be used to increase the utility of MRI in research and for diagnostics and therapeutic uses for patient health care.